/Engine/Plugins/Runtime/GeometryProcessing/Source/DynamicMesh/Public/Polygroups/PolygroupSet.h
Set the PolygroupID for a TriangleID
void SetGroup ( int32 TriangleID, int32 NewGroupID, [FDynamicMesh3](API\Runtime\GeometryCore\DynamicMesh\FDynamicMesh3) & WritableMesh )